The present invention is generally directed to processes for the preparation of toner resins and toners. More specifically, the present invention relates to melt mixing processes, batch or continuous, but preferably continuous processes such as, for example, non-reactive extrusion for preparing toner resin blends containing, in embodiments, mixtures of two partially crosslinked resin, and in other embodiments, mixtures of a partially crosslinked resin and an uncrosslinked resin, wherein the resulting toners have optimized gloss properties, broad fusing latitudes, and extended fuser roll life.
Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, numerous polymers are known, including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black, magnetite and various colored pigments may be selected, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, metallic alkyl salicylates, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or a transparency, hot roll fixing is commonly used. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll or belt. Upon contact with the heated fuser roll or belt, the toner melts and adheres to the support medium, forming a fixed image. Such a fixing system is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is called the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is called the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred or extraneous images. This undesirable phenomenon is called offsetting. Between the COT and HOT of the toner, is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is called the Fusing Latitude, and the temperature range therebetween is referred to the fusing range.
The hot roll or belt fixing system and a number of toners used therein, however, exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption. Low vinyl type binder resins such as styrene-acrylic resins may have an additional problem known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with, for example, a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl notebook binder covers, and the fixed image adheres to the PVC surface. Another problem, particularly for highlight and process color applications is the inability to readily form images which have variable or operator selectable gloss properties, for example, images having colored regions with high gloss levels and black regions, for example text regions, with low or intermediate gloss levels.
In the aforementioned copending U.S. Ser. No. 08/393,606 is disclosed a process for the preparation of pigmented toner compositions comprising: forming at a first temperature, a first melt mixture comprised of a partially crosslinked thermoplastic resin, pigment, optionally a wax, and optional additives, wherein the partially crosslinked thermoplastic resin is comprised of a mixture of crosslinked resin macrogel particles, crosslinked resin microgel particles, and uncrosslinked resin; and melt mixing at a second temperature, the first melt mixture to form a second mixture, wherein the macrogel particles are partially converted into microgel particles, and wherein the second temperature is less than or equal to the first temperature.
In the aforementioned commonly assigned U.S. Pat. No. 5,312,704, are disclosed imaging processes and toner compositions comprised of pigment particles, and a resin, for example, anionically polymerized styrene-butadiene, comprised of a monomodal polymer resin or monomodal polymer resin blends, and wherein the monomodal resin or resin blends possess a narrow polydispersity. Also disclosed are toner compositions comprised of monomodal toner resins with low, high, or intermediate gloss properties. The high weight average molecular weight monomodal resins generally have low gloss properties, the low weight average molecular weight monomodal resins generally have high gloss properties, and the intermediate weight average molecular weight monomodal resins or blends of low and high weight average molecular weight monomodal resins generally possess gloss properties intermediate between the aforesaid high and low gloss properties.
In order to prepare lower fix temperature resins for toner, the molecular weight of the resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used to prepare low temperature fixing toners. For example, attempts to produce toners utilizing polyester resins as binder are disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be rendered lower than that of other materials, such as styrene-acrylic resins. However, this may lead to a lowering of the hot offset temperature and, as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll or belt offsetting and to increase fusing latitude of toners, modification of the binder resin structure by conventional polymerization processes, for example, by branching, cross-linking, and the like, has been attempted. For example, in U.S. Pat. No. 3,681,106 to Burns et al., a process is disclosed whereby a polyester resin was improved with respect to offset resistance by non-linearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is by cross-linking during polymerization. In U.S. Pat. No. 3,941,898 to Sadamatsu et al., for example, a cross-linked vinyl type polymer prepared using conventional cross-linking was used as the binder resin. Similar disclosures for vinyl type resins are presented in U.S. Pat. No. Re. 31,072 (a reissue of U.S. Pat. No. 3,938,992) to Jadwin et al., U.S. Pat. No. 4,556,624 to Gruber et al., U.S. Pat. No. 4,604,338 to Gruber et al., and U.S. Pat. No. 4,824,750 to Mahalek et al. Also, disclosures have been made of cross-linked polyester binder resins using conventional polycondensation processes for improving offset resistance, such as for example in U.S. Pat. No. 3,681,106 to Burns et al.
While significant improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue with these kinds of cross-linked resins prepared by conventional polymerization, including solution, bulk, suspension and emulsion polymerizations and polycondensation processes. In all of these processes, monomer and cross-linking agent are added to the reactor at the same time. The cross-linking reaction is not very fast and chains can grow in more than two directions at the cross-linking point by the addition of monomers. Three types of polymer configurations are produced; a linear and soluble portion called the linear portion; a cross-linked portion which is low in cross-linking density and therefore is soluble in some solvents, such as, tetrahydrofuran, toluene, and the like, and is called sol; and a portion comprising highly cross-linked gel particles which is not substantially soluble in any solvent, for example, tetrahydrofuran, toluene and the like, and is called gel. The second portion with low cross-linking density (sol) is responsible for widening the molecular weight distribution of the soluble part which results in an elevation of the minimum fixing temperature of the toner. Also, a drawback of these processes, which are not carried out under high shear, is that as more cross-linking agent is used the gel particles or very highly cross-linked insoluble polymer with high molecular weight increase in size. The large gels can be more difficult to disperse pigment in, causing unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, in the case of vinyl polymers, the toners produced often show vinyl offset.
In U.S. Pat. No. 4,533,614 to Fukumoto et al., a process was utilized for preparing loosened cross-linked polyester binder resin which showed low temperature fix and good offset resistance. Metal compounds were used as cross-linking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 to Burns et al. and Japanese Laid-open Patent Applications Nos. 94362/1981, 116041/1981 and 166651/1980. As discussed in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner.
In U.S. Pat. No. 5,241,020, to Roha, issued Aug. 13, 1993, is disclosed novel blends of polymers produced by polymerization of interactive compounds that form polymers in a non-free radical polymerization, and at least one monomer possessing carbon-to-carbon double bonds capable of polymerization by means of a free radical mechanism, in the presence of reactive initiators. In a preferred mode, a reaction mixture is formed comprising the interactive compounds, the monomers, and the reactive initiators. The interactive compounds are reacted in an initial step to form a first polymer connected to the reactive initiator. In a subsequent reaction free radicals derived from the reactive initiator promote the polymerization of the monomers to form a second polymer. The first polymer forms the continuous phase of the blend, while the second polymer comprises the discontinuous phase. As a consequence of the enhanced inter-phase adhesion resulting from the interaction of the reactive initiators with both polymers, the blends display superior tensile strength, elastic recovery and increased elongation at break.
In U.S. Pat. No. 5,057,392, to McCabe, issued Oct. 15, 1991, is disclosed a low fusing temperature toner powder comprising a polyblend of a crystalline polyester and an amorphous polyester which are crosslinked with an epoxy novolac resin. The crystalline polymer melts at a relatively low temperature and has a relatively low glass transition temperature, while the amorphous polymer has a high glass transition temperature. The crystalline polyester has a number average molecular weight in the range of about 1,000 to about 3,000 and a weight average molecular weight in the range of about 2,000 to about 6,000. The amorphous polyester has a number average molecular weight in the range of about 1,000 to about 3,000 and a weight average molecular weight in the range of about 2,000 to about 9,000.
Thus, there remains a need for toner resins with low fix temperatures and high offset temperatures or wide fusing latitudes, superior or nonexistent vinyl offset property, efficient and economic processes for the preparation of such resins, and the ability to control the gloss levels and gloss properties of the resulting toners and printed images formed therewith.
There is also a need for black or colored toners wherein the aforementioned properties are controllable and preferably selectable during formulation. There is also a need for black and colored toners that are non-blocking, such as from about 115.degree. F. to about 120.degree. F. (46.1.degree. to 48.9.degree. C.), of excellent image resolution, non-smearing, and of excellent triboelectric charging characteristics. In addition, there is a need for black or colored toners with low fusing temperatures, of about 110.degree. C. to about 150.degree. C., of intermediate gloss properties, in embodiments, such as from about 10 to about 25 gloss units, and in other embodiments, from about 40 to about 70 gloss units, of high projection efficiency, such as from about 75 percent efficiency to about 95 percent efficiency or greater, and toner compositions that result in developed images with minimal or no paper curl or fuser roller hot offset.